Fluid inclusions in quartz from a quartz breccia in the Siljan Ring structure, central Sweden

The Siljan structure represents a deep erosion level of an astrobleme. Post-impact formation of minerals as various types of vein fillings is common. Non-shocked milky quartz from a large vein revealed fluid inclusions of three types:(1) one-phase, regularly shaped inclusions, < 1 to 10 μm in size, (2) two-phase, regularly to irregularly shaped inclusions, 2 to 7 μm, (3) larger irregular two-phase inclusions, 8–20 μm. The one-phase inclusions dominate; density of inclusion occurrence is estimated at 46 × 104 inclusions per cm2. Melting temperatures indicate a total salinity of 1.0 eq. wt. % NaCl. Homogenization temperatures range between 165 and 309°C. The fluid inclusion data imply a complex origin of the quartz. It consists in part of breccia fragments formed after the impact. The quartz vein was cemented by deep circulating meteoric waters at successively lowered temperatures initially over 300°C.